1. Field of the Invention
The present invention relates to diamond-like carbon film (DLC film for short hereinafter) and a method for production thereof. Because of its good durability and low frictional coefficient, DLC film is used as hard coating film that is applied to sliding parts, such as automotive cams and shims, which slide over surfaces between two parts in contact with each other. The present invention relates also to a sliding part coated with such hard coating film.
2. Description of the Related Art
DLC film to be applied to the surfaces of sliding parts should have several important characteristics, one of which is good adhesion to the substrate. Many inventions (mentioned later) have been disclosed about improvement in adhesion from the standpoint of the composition and forming method of DLC film.
Patent Document 1 is concerned with a protective film for recording media which is formed by plasma CVD. This protective film has a disadvantage of requiring an intermediate layer thereunder if the substrate is stainless steel for which a thick DLC film is not suitable. The intermediate layer containing carbon is intended to improve adhesion between the DLC film and the substrate.
Patent Document 2 is concerned with a nitrogen-containing chromium film, which is intended to improve adhesion between DLC and a substrate of alloy steel. This idea was conceived to overcome weak adhesion due to difference in hardness, expansion coefficient, and structure between the substrate and the DLC film. (The DLC film in this case includes the one composed of hard and soft layers placed one over another.)
Patent Document 3 is concerned with articles with a protective DLC film. This DLC film is intended to eliminate pretreatment, such as ion etching, which is necessary to provide a high bonding force to the DLC film in the case where the substrate is steel, alloy, glass, or ceramics. The object of improving adhesion is achieved by forming an intermediate layer by ionization vapor deposition. This intermediate layer has a hardness of 1000 to 5000 Hv, so that it is softer than the substrate and harder than the DLC film.
Patent Document 4 is concerned with a multi-layered structure for improved wear resistance and adhesion, which is composed of a metal layer, a metal/carbon gradient layer, and a DLC layer having a region in which hardness varies. This idea was conceived to eliminate difficulties in providing a strong, sound DLC film, which was not achieved by the conventional W-containing DLC layer or amorphous carbon to exhibit good wear resistance and adhesion for sliding parts.
Patent Document 5 is concerned with a composite hard coating film composed of intermediate layers of nitride, oxide, and carbide (which are formed by PVD), a layer of Si compound, and a layer of Si-containing DLC. This composite coating film both improves adhesion and lowers frictional coefficients.
The technologies of DLC film disclosed in these Patent Documents have in common one object of improving adhesion with the substrate. However, all of them have the following problems in view of the technical background of the present invention.
The inventions disclosed in Patent Documents 1, 2, or 3 propose the metal layer or the gradient structure of metal and carbon layer for improvement in adhesion. The intermediate layer of such structure is effective for good adhesion but has a low hardness. Therefore, when used under a high contact pressure, the coating film breaks at that part of the intermediate layer which undergoes plastic deformation. This poses a problem with durability.
The invention disclosed in Patent Document 4 is intended to improve adhesion by providing an intermediate layer which has a region in which hardness varies. This structure inevitably contains a region in which hardness is low; therefore, it poses a problem with durability for use under a high contact pressure of sliding.
The invention disclosed in Patent Document 5, which is concerned with a method for forming intermediate layers of nitride, oxide, and carbide by PVD, and subsequently forming thereon a layer of Si compound and an Si-containing DLC layer, has a disadvantage of requiring a long processing time because of increased steps even though each intermediate layer can be adjusted for hardness.    Patent Document 1:
Japanese Patent Laid-open No. 2000-256850    Patent Document 2:
Japanese Patent Laid-open No. 2007-31797    Patent Document 3:
Hei-5-117087    Patent Document 4:
Japanese Patent Laid-open No. 2004-10923    Patent Document 5:
Japanese Patent Laid-open No. 2003-268571